The Occupational Safety and Health Administration yesterday ordered a fabric-coating firm to pay almost $500,000 in fines, saying that more than half of the firm's employes showed signs of overexposure to a liver-attacking chemical.

OSHA, the federal agency responsible for protecting the safety of U.S. workers, accused Uretek Inc., a New Haven, Conn., firm, of 34 "willful" and 145 "serious" violations of federal safety laws.

It was the latest crackdown by OSHA, which has been severely criticized for relative inactivity during the Reagan administration.

The firm refused to comply with rules on exposure to chemical solvents, including the potent liver toxin dimethylformamide, the agency said.

OSHA said 34 of the firm's 65 employes showed signs of overexposure to the chemical.

Uretek officials were not available for comment.

"The company's continued failure to ensure the use of personal protective equipment for employes exposed to dimethylformamide, which can be readily absorbed through the skin, demonstrates a serious disregard for the health of those workers," OSHA chief John Pendergrass said in a statement.

OSHA ordered the company to pay penalties totaling $480,840. The 34 "willful" penalties each carried a $10,000 maximum penalty.

"The company's accident and illness records for calendar years 1985 and 1986 showed six reported cases of liver abnormalities -- three for each year -- and additional cases were found by inspectors," an OSHA spokesman said.

"In all, 34 employes showed signs of overexposure to the liver toxin," OSHA said, adding that "little effort was made to train the employes in proper methods of personal protection" from chemicals.

The company knew of the liver abnormalities as early as November 1986, OSHA said, when a medical clinic examined an employe who complained of illness.

"The clinic later advised the company on ways to protect the workers, but many of the suggestions were never implemented," OSHA said.

The agency said the 34 employes who work with chemical solvents were not required to use gloves, aprons or foot coverings. Also, OSHA said, employes were allowed to store food and beverages in contaminated areas.

Uretek uses a variety of chemicals to coat synthetic fabrics. The company has 15 working days to pay the fines or challenge OSHA before the Occupational Safety and Health Review Commission.

On July 21, OSHA proposed a $2.59 million fine against IBP Inc., the nation's largest meatpacker and a subsidiary of Occidental Petroleum Corp. It was the largest fine ordered by OSHA in its 16-year history.